Electron discharge device with indirectly heated cathode



Feb. 6, 1934. E. A. LEDERER ET AL 1,945,746

ELECTRON DISCHARGE DEVICE WITH INDIRECTLY HEATED CATHODE Filed Nov. 161927 INVENTOR zA/EsT ,4. ZEDEEEE. LTOHN W MAEOE/V.

ATTORNEY Patented Feb. 6, 1934 Emerson nrsonaaen :osvros WITH IN-DIEEC'ELY HEATEB CATHODE Ernest A. Lederer and John W. Marden, EastGrange, N. 5., assigncrs to Westinghouse Lamp Company, a corporation ofPennsylvania Application November 16, 1927 Serial No. 233,537

3 Claims.

This invention relates to an electron discharge device and moreparticularly to such device of the type in which the cathode is heatedindirectly by a heating element disposed adjacent thereto.

6 In the manufacture of electron discharge devices in which the cathodeis designed to be heated through the agency of alternating current, ithas been the usual practice to construct the oathode in theform ofahollow metal cylinder and to 10 heat the same by conduction from anelectrically insulated heating element. The heating element of suchcathode usually consists of a tungsten filament which is supportedwithin an aperture in a, cylindrical insulator about which the hollowmetal cylinder, coated with a thermionically active material such as theoxides of the alkaline earth metals, is positioned. This coated cylinderconstitutes an equi-potential cathode and may be provided with aterminal separate from those of the heating element.

In constructing cathodes of this type, difiiculty has been encounteredheretofore, due to interaction of the tungsten filament with theinsulator and to the fusing of the insulator on to the filament,resulting in early burn-out of the heating element. This difliculty hasbeen overcome to a large extent by the use of thorium oxide, zirconiumoxide or rare earth oxide in place of the usual porcelain or Isolantiteinsulator, as is more fully set forth in copending application of JohnW. Marden and Frank H. Driggs, Serial No. 233,563, filed November 16,1927 and entitled, Insulating material for vacuum electric devices.

The method which is used very largely and which is preferred by us forproducing the thermionically active coating on the metallic cylinder isto first coat the cylinder with the carbonates of the alkaline earthmetals, which are baked thereon in a firmly adherent manner anatmosphere of carbon dioxide. After the oathode has been assembledwithin the evacuated device, the cylinder is heated to a hightemperature to convert the alkaline earth carbonates into oxides of thealkaline earth metals. It has not been found advisable to effect thisconversion of the carbonates to the oxide before assembly of the cathodein the device, since the oxides apparently become contaminated in airand the electron emission of the cathode is impaired and heretofore,this heating of the metal cylinder has been accomplished by passing aheavy current through the heating elements to raise the same to anabnormal temperature.

However, due to the high temperature to which the heating element mustbe raised to heat the surrounding cylinder to thedecompositiontemperature of the carbonates, a chemical reaction appearsto take place between the tungsten and the insulator when the same ismade of porcelain .60 or Isolantite, possibly due to interaction betweenthe tungsten and water vapor, carbon dioxide or other gas liberated fromthe insulator. The insulator also fuses on to the tungsten heater wireduring this heat treatment and upon cooling of 5 the cathode assembly, aportion of the tungsten adheres to the porcelain or Isolantite and ischipped off from the tungsten body, due to the diiierence in contractionof the insulator and the tungsten filament. As a result of thesedifliculties, the filament burns out prematurely and the life of thedevice is relatively short.

One of the objects of the present invention is to provide a constructionin which the above mentioned difiiculties will be avoided and in whichconversion of the carbonate coating on the cathode may be effectedwithout necessity of employing the enclosed heating element.

Another object is to provide a construction in which the use of solidinsulating material, such so as porcelain, between the cathode and theheating element may be entirely dispensed with.

A further object is to provide a construction in which the electrodeswill be shielded from electrons emitted from all portions of the heatingeles5 ment.

A further object is to provide a simple and rigid support for theelectrodes which will enable the cathode to be heated by high frequencyinduction current without substantial heating of the other-'90electrodes.

Other objects and advantages will hereinafter appear.

In accordance with the present invention, we construct the cathode inthe form of a hollow metallic member having a coating of athermionically active material on the exterior thereof and a heatingelement contained therein. The heating element may be supported within arefractory insulating member contained within the cathode so as toheatthe same by conduction or the refractory insulation may be entirelyomitted and the cathode heated by radiation from the heating element. Ifdesired, the heating element may be operated at an electron emittingtemperature and a difference in potential may be maintained between theheating element and the cathode to cause the cathode to be heated byelectron bombardrnent from the heating element.

The cathode is arranged so as to form one por- 11 however, such tion ofa closed loop in which the currents may be induced by a high frequencycoil disposed outside of the envelope. This closed loop is arranged insuch position that upon heating thereof by high frequency inductioncurrent, heating currents are not set up in other and undesired portionsof the electrode assembly.

With this construction, it is possible to heat the cathode duringexhaust, to a high temperature to decompose the carbonates of thealkaline earth metals thereon and to convert the same into oxides of thealkaline earth metals without heating the filament and having undesiredreaction with carbon dioxide or other gases iormed during the operation.

Shields are disposed about the upper and lower ends of the cathode toprevent stray or undesired discharges taking place between the heatingelement and the control electrode or anode which might result inalternating current hum when the :device is employedin radio receivingapparatus.

In order that the invention may be more fully understood, reference willbe had to the accompanying drawing in which:

Fig. 1 represents an electron discharge device partly in sectionembodying the present invention;

Fig. 2 is a sectional View taken on line II-II of Fig. 1;

Fig. 3 is a fragmentary view showing a modified form of cathodeconstruction; and,

Fig. 4 is a sectional view of a further modified form of cathodeconstruction.

The electron discharge device shown in Fig. 1 comprises a hermeticallysealed envelop having an electrode assembly 11 therein composed of acathode 12, grid or control electrode 13 and anode 14. The cathode 12 isin the'form of a hollow metal cylinder, preferably of nickel having onthe exterior surface thereof a coating 15 of the oxides of the alkalineearth metals or other thermionically active material. Annular discs 16and 17 surround the upper and lower ends respectively of the cathode 12and are rigidly secured thereto by a number of radially extendingsupporting strips 18. The discs 16 and 17 serve to support the cathodefrom the support wire 19 through intermediate supporting strips 20welded to the discs and to the support wire. The support wire 19 at itslower end is secured to a rigid wire 21 sealed in the press 22 of thedevice and joined to leading-in conductor 23. At the upper end thesupport wire 19 is welded to two bracing supports 24 and 25 secured in aglass bead 26 positioned above the electrode assembly.

A heating element 27 which may taken the form of an inverted V-shapefilament of tungsten or other highly refractory metal, having theadjacent legs arranged to neutralize the electric fields set up therein,is disposed within the cylindrical cathode 12 and is supported at itsloop portion by a resilient member 28, having one end secured in theglass bead 26. The free ends of the filament are joined to support wires29 and 30 which are connected to the leading-in wires 31 and 32respectively for supplying heating current thereto. Other forms ofheating element may be employed, as a helical filament.

A grid electrode 13 consists of a helically wound wire, the adjacentturns of which are secured to a grid support wire 33, mounted on therigid support wire 34 by the intermediate strips 35 and 36. The supportwire 34 is secured at its upper end to the bead 26 and at its lower endis sealed in the press and joined to the leading-in conductor 37.

The plate electrode or anode 14 consists of a metallic cylinder having aradially extending flange 38 to which a support wire 39 is welded orotherwise suitably secured. The upper end of the wire 39 is secured inthe bead 26 and the lower end thereof is sealed in the press 22 andjoined to the leading-in conductor 40.

The leading-in wires 23, 31, 32, 37 and 40 are electrically connected tothe contact pins 41 carried by the base 42.

The annular discs 16 and 17 serve to protect the grid 13 and anode 14against a flow of electrons from the exposed ends 43 and 44 of theheating element. We have found, due to the high temperature at which theheating element is operated, that electrons are emitted therefrom anddrawn to the other electrodes if some shielding means is not provided.This electron fiow sets up a fluctuating current in the output circuitof the device which, in many cases, is detrimental to its operation. Thediscs 16'and 17 may be composed of metal or a suitable insulatingmaterial such as mica. If they are composed of insulating material itis, of course, necessary to extend the supporting strips 20 over: to thehollow cylinder 12 to conduct the current to the cathode from thesupport wire 19. Obviously, in place of making the members 16 and 17 ofdisc form, they may be conical or any other suitable shape which willprovide a barrier be- 105 tween the exposed ends of the heating elementand the anode and control electrode.

The supporting members 20, discs 16 and 17, support wire 19 and cylinder12 form a closed loop. The cathode 12 should be formed of thin;-1-l0sheet metal and the remaining portions of the loop of sufiiciently heavymaterial to render it possible to heat up the cathode cylinder to a hightemperature by means of a high frequency induction coil disposedexternally of the enve-. 115 lope and arranged with its axissubstantially normal to the bulb. This permits the oxide coating on thecathode to be formed from a coating of the alkaline earth carbonatesafter the cathode is incorporated in the exhausted;;120 envelope withoutnecessitating the heating of the tungsten filament 27.

Of course, if desired, the shields 16 and 17 may be omitted in whichcase the cathode cylinder 12 may be supported directly from the wire 19byip-l25 the supporting strips 45 and 46 as shown in Fig.

3 so as to form the closed loop with the cathode. In Fig. 4 a modifiedform of cathode is illustrated in which a solid insulating member 47 ispositioned within the cathode 12 and has a pairzz130 of aperturestherein through which the heating element 27 extends, whereby thecathode is heated by conduction rather than radiation. In the form shownin Figs. 1, 2 and 3 in which the insulating member 47 is not employed,the cathode 120135 may be heated by electron bombardment from theheating element 27 by maintaining the heating wire negatively chargedwith respect to the interior surface of the cylinder.

It is obvious, of course, that many changes may: be made in theconstruction shown and described and we do not desire to be limitedthereby but desire the described embodiments to be construed in allrespects as illustrative and not restrictive, reference being had to theappended I45 claims to indicate the scope of the invention.

What is claimed is:

1. An electron discharge device comprising an evacuated envelope, ahollow thermionically active uni-potential cathode and an anode, anelec- 3. An electron discharge device comprising an evacuated envelope,a tubular cathode therein, a,

heating element within said cathode, a closed electrically conductiveloop within said envelope;

including said cathode, whereby said cathode may be inductively heatedfrom an external source, an annular dielectric shield surrounding eachend of said cathode and a plurality' of electrodes disposed about saidcathode between said shields.

ERNEST A. LEDERER.

JOHN. W. MARDEN.

